In mobile telecommunication electromagnetic waves in the microwave region are used to transfer information. An essential part of the telecommunication device is thus the antenna, which enables the reception and the transmission of electromagnetic waves.
Cellular systems of the 2nd generation operate in two different frequency bands called GSM (Global System for Mobile) and DCS (Digital Communication System). In Europe the frequency bands GSM 900, which is located at 880 MHz to 960 MHz, and GSM 1800 (DCS), located at 1710 MHz to 1880 MHz, are used. Additionally there is the GSM 850 frequency band from 824 MHz to 894 MHz and the GSM 1900 (PCS) frequency band from 1850 MHz to 1990 MHz mainly used in the United States.
Wireless communication devices operating in two or more frequency bands, for example those which work in the GSM and DCS/PCS frequency bands, need one or more filters to split the signals of a radio frequency front-end into a GSM path and a DCS/PCS path. For this purpose active or passive electronic circuits or complex filter units such as diplexers (or duplexers to separate between the transmitting and the receiving sub-bands) can be used. These filters are connected to the antenna, and serve to switch from one frequency band to another.
In order to satisfy the growing trend towards miniaturization of wireless communication devices efforts have been made to improve and simplify these filters such that they can be made smaller. EP 1 119 069 A2 for example discloses a diplexer of which the flexibility of the frequency shift degree is high and which has a small size.